There are so many SBC out there it really depends on what are your core requirements?

I am looking for something similar to Arduino UNO, same popularity (or higher), but different board or different chip manufacturer e.g. Arduino Nano, NXP, ST Micro, etc. I do not need it for any particular project. It is a board to play with - I would like to have a look at a board what many people use to build their own projects.

The raspberry pi is more of a SBC, and there have been discussions where people were asking why there are no clones, or wondering if they can build something similar, and because of licensing on the broadcom chip and HDMI, it's pretty much a black box component that is only available on the rPi

One thing I have to mention though about the Bluepill: When I did my bachelor-project earlier this year, i2c seemed to be running in Software. Symptoms of that were a max. SCL-Frequency of about 170kHz, as well as a complete lockup of the chip if a certain timer (can't remember which one) was being used by the application when i2c-Communication was also being implemented.

Another big problem I found is the fact that there are no asynchronous timers in the STM32F103-Series. Makes measuring of high frequencies difficult to impossible and the timer-functions in general had limited easy to understand documentation and examples available. The latter might have improved, as well as the i2c-situation, but those were stil two problems I was only able to solve by adding a Arduino Nano about 3 weeks before the deadline to one of the expansion-slots I had put into the design for exactly this kind of eventuality.

Current project I use this chip in is my RF Signal-Generator. Even with a graphical user interface and totally non-optimized software, it has plenty of memory left and over 30MHz SPI-Clock to program signal sources and send data to the display. Once I understand how to program the DMA, or the libraries for the display start making use of DMA, things can probably be sped up quite considerably. So a lot of potential in that chip to be unlocked if programmed with the Arduino IDE and the Libraries of the STM32Duino-Project

Thank you for suggestions. I checked Teensy and NodeMCU and there is not really much information about licensing. Maybe I missed something?

I was thinking about Raspberry Pi, but I believe, RPi is not really open source and also it may be difficult to buy some of the chips.

Neither Teensy, NodeMCU or RPi are Open Source, if by Open Source you mean both the hardware and supporting software are published with a recognized Open Source license.

If you exclude Arduino Uno, it's very difficult to find any boards that are both popular and Open Source. Since no one really knows how many boards are sold, deciding if a board is "popular" is largely subjective.

Arduino is unique in being the only genuinely Open Source board. Why did you exclude it anyway?

Arduino is unique in being the only genuinely Open Source board. Why did you exclude it anyway?

I excluded just Arduino UNO. I have created a lot of materials based on Arduino UNO, so I am looking for a different board now. Arduino Nano is a very good candidate + I am also considering something with STM32. I am just not convinced, that community around STM32 is as as strong as around Arduino Nano.

None of these boards are something you would use in a project. These boards are basically just a breakout board with some passives. You will generally always want to use the microcontroller directly and avoid a lot of cost and manufacturing headache (soldering pin headers is labor intensive and to be avoided).

A stm32f103 needs no external components other than a bypass cap (crystal is optional). I would imagine it's similar with most other modern microcontrollers these days. There is no excuse to not directly put it on your board.

Even in the prototyping phase I never start with an arduino or other dev board. The circuit will differ so much compared to your final PCB that the tests done on an arduino are meaningless. I have designed my first PCB that includes an stm32 without having ever touched a dev board (I have looked at its schematic though).

If we are talking SBCs then it's a different story, but if you are making a product it still makes more sense to make your own board since BGA and multilayer boards add very little cost in production compared to hand soldering pin headers.

None of these boards are something you would use in a project. These boards are basically just a breakout board with some passives. You will generally always want to use the microcontroller directly and avoid a lot of cost and manufacturing headache (soldering pin headers is labor intensive and to be avoided).

This is perfectly fine. I just want to re-design the board a little bit and make some videos about how to add your own changes. Then, from the videos people should be able to learn how to customize this board, re-design it to their needs and for their own applications.

SparkFun created an ATmega32U4 board, Pro Micro under CC-SA 4.0. The ubiquitous cheap clones use the Arduino Leonardo bootloader. I like the ATmega32U4 because of its simplicity and hardware USB 1.1 (12 Mbits/s, easily reaches a megabyte a second using e.g. USB serial endpoint). The HID support in particular is very nice: makes it very easy to create human interface devices like keyboards, joysticks, and gamepads. (I've also used one to simulate an USB-connected vinyl cutter I did not have access to, but had an USB data dump for, for developing/bug-fixing a driver for.)

With the Leonardo bootloader, in Linux, the best way to get them reliably programmed is to detach, wait a couple of seconds, then reinsert, wait a second, and click program in the Arduino environment; the timing is sometimes a bit tricky to get right. (Getting the Leonardo bootloader to properly trigger a firmware upload is the issue.)

You can obviously create your own bootloader, too, but I've not bothered yet. There is even an instructable, IIRC, on how to upload a new bootloader to a Pro Micro clone. If you want to play with making your own board variant, you could start with the Pro Micro at EasyEda (an online circuit design tool).

As to Teensies, head over to PJRC.com or the forum there. Paul Stoffregen is the developer of Teensies. To create your own Teensy variant, you can create your own board based on the schematic (the board files aren't publicly available, I think), and buy a preprogrammed bootloader chip from PJRC.com. This is what others have done. PJRC.com funds the development by keeping the bootloader (including processor initialization part) proprietary. Note that Paul Stoffregen not only develops the Teensies and maintains Teensyduino, he also supports the Arduino environment, both via fixing bugs and developing various libraries (at GitHub).

(I personally could be described as a rabid open source and linux advocate, but for cases like this, I definitely understand why keeping some parts proprietary is necessary for a very small company to fund the development.)

Note that Paul Stoffregen not only develops the Teensies and maintains Teensyduino

Thank you.

Pro Micro - I like the direct USB and I am checking now also Arduino LeonardoTeensies - I had a look at these, but as the licensing is unclear, it just would not feel right to create a clone. From all the boards I had a look at, the Arduino is still on the top of my list, but I keep looking.

PS: For my initial testing I bought Arduino Vidor and played with it. That board is in very early stage of product cycle at this moment - not much documentation and software around it yet

Well, if you need more than the simplest micro, such as something that runs apps on the net, the Beagle Bone is quite good. 1 GHz Arm processor, ethernet, lots of I/O, USB host and target, and a pair of 200 MHz microcontrollers for things that need fast "real time" interfacing. Certainly more expensive than an Arduino, but a heck of a lot more powerful. I've used them for several projects that needed to be remote-controlled over the net via some kind of GUI.